Circular  No.  106. 


' 


I  nited  States  Department  of  Agriculture, 

BUREAU  OF"  KNTOMOI.OGY. 


u    HI 


. 


itomologUt  iin.l 


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miml,    Rile)  \^    j£ 


Till:  WHEAT 

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i ii  c    I    Rebvbs, 
•eal  and  I 

wheal  strawworm  (fig.  I    sustains  about  the  same  relations  to 
winter-wheat  culture  west  of  the  Mississippi  River 
thai   the  joint  worm  (Isosoma  tritici  Fitch   '  does 

to  the  Cultivation  Of   this  cereal  east   of  this  river. 

Both,  when  excessively  abundant,  occasion  losses 

Taiying  from  slight  to  total  A  wheat  stem  at- 
tacked by  the  joint  worm  may  produce  grain  of  a 
more  or  less  inferior  quality  and  less  of  it ;  but  the 
spring  attack  of  the  wheat  Btrawworm  (fig.  5)  is 
fatal  to  the  plant  affected,  as  no  grain  at  all  is  pro- 
duced; and  while  the  second  generation  of  the 
>;inie  has  a  less  disastrous  effect  in  the  field,  it 
nevertheless  reduces  the  grade  and  weight  of  the 
in. 

In  the  Ohio  Valley  and  south  of  Pennsylvania 
the  ranges  of  these  two  insects  overlap  (fig.  _' 
both  species  are  often  to  be  found  in  the  same 

held,  the    wheat    strawworm,  however,  being    less 

abundant  and  doing  usually  but  slighl  injury, 
while  the  joint  worm  occasionally  becomes  a  seri- 
ous pest. 


uing  the  last  two  yean  Mr   Reeves  ha 
in  the  investigation  of  this  peal  in  the  State  of  Was! 
in  cooperation  with  the  agricultural  experiment   station  of 
thai   State.     With  the  exception   of   the  - 
relative  to  the  l>..|i.i\i.>r  of  the  peel  in  the  Northwest,  for 
which  exclusive  credit  i-  to  i  the  ji 

senior  author  i-  reeponsihle  for  the  circular 
►  See  Circuit  Bureau  i  '        Tnu 

culture.   I  I 

B1572-  Cir. 


: 


fmitmmttjot  Agri- 


Wesl  of  the  Mississippi  River,  throughout  the  winter-wheat  grow- 
ing territory,  the  jointworm,  if  it  occurs  at  all,  is  never  destructive. 
The  wheat  strawworm,  on  the  other  hand,  appears  to  he  generally 
distributed,  at  times  committing  very  serious  depredations  which 
have,  at  least  in  some  cases,  been  charged  to  the  Hessian  fly,  as  shown 
by  the  fact  that  some  of  these  ravages  have  occurred  hi  sections  where 
the  Hessian  fly  is  not  known  to  exist.  Besides,  while  the  Hessian  fly  is 
manageable  with  more  or  less  difficulty,  the  wheat  strawworm  is 


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S.     \   *   ASOSOHW      Tf?/r/C/  fitch.      V,  I 

Fig.2.     Present  known  distribution  <>f  Isoaoma  grandU  and  Tsosoma  triti<i  in  the  United  Slates. 

(Original,  i 

one  of  the  easiest  of  all  insect  pests  to  control  by  entirely  practical 
measures.      Indeed,  it  need  not  be  allowed  to  become  a  pest  at  all. 

DESCRIPTION'S    AND    SEASONAL    DEVELOPMENT. 


There  are  two  generations  of  the  insect  annually,  the  adults  of  the 
first  generation  (fig.  3)  differing  considerably  in  appearance  from 
those  of  the  second  (fig.  4).  To  the  farmer  they  will  all  look  like 
minute  or  large,  shining  black  ants,  with  or  without  wings,  their  legs 
more  or  less  banded  with  yellow,  and  having  red  eyes.  Individuals 
of  the  first  generation  emerge  in  April  from  the  outstanding  straws 
and  stubble.  They  are  very  small,  most  of  them  are  females,  and 
many  are  wingless.  The  females  deposit  their  eggs  in  the  young  wheat 
plants,  the  stems  of  which  at  this  time  extend  but  little  above  the 
surface  of  the  ground.  The  egg  is  placed  in  or  just  below  the  embry- 
onic wheat  head  (see  fig.  (i)  and  the  larva  or  worm  works  within  the 
[Cir.  106] 


3 


Btem,  HsnalK  causing  a  slight  enlargement.  When  the  worm  is  full 
grown  it  will  be  found  in  the  crown  of  the  plant,  having  eaten  out 
i,n,l   totally    destroyed  the  embryi  "I.  its 

bodj   occupy  ing  i  he  ca^  itj   t  hua  formed. 

The  females    !'iLr.  3  I  which  deposit   these  e 
being  small  and   frequentlj   wingless,  are   in  no 
v.;,\    fitted    for    traveling    long   distances.     The 
larva  or  worm  of  a   verj  light   stra^ 

color,  indeed  almosl  white,  with  brown  jaws,  the 
form  of  the  bod}  being  as  shown  in  the  illustra- 
tion. These  worms  develop  \er\  rapidrj  and, 
iis  they  feed  on  the  most  nutritious  pari  of  the 
plant,  they  become  robust  and  larger  than  those 
found  iii  the  mature  straM  in  late  summer.  In 
Ma\  the  larva  become  fullgrown  (fig.  5) and  pass 
at  .nice  through  a  short  pupal  9tage.  (See  fig.  7. 1 
The  pupa'  are  at  first  the  same  color  as  the  larvse, 
but  later  change  to  a  shining  jet  black.  En  a  fVw 
days  the  fully  developed  insects  gna\*  circular 
holes  through  the  wall-  of  the  stem  and  make  their  waj  out.  These 
adults  (fig.  1 1  are  much  larger  and  more  robust  than  the  individuals  of 


■lull  ul 
mm  of  f'Oionnj 
il.) 


Fu..  I 


the  lir-t  generation  and  arc  provided  with  fully  developed,  serviceable 
wings.  That  they  make  good  use  "1*  their  wings,  and  scatter  them- 
Belves  aboul  over  fields  adjacent   to  their  place  of  development,  ia 

Kir.  106] 


shown  by  their  occurrence  in  fields  of  grasses  (in  the  stems  of  which 
they  do  not  breed)  situated  considerable  distances  from  wheat  fields. 

In  ovipositing,  the  females  of  this  generation  select  the  largest  and 
most  vigorous-growing  stems  in  which  to  place  their  eggs. 

The  adults  of  the  second  generation  deposit  their  eggs  from  early 
May,  in  Texas,  up  to  the  middle  of  June,  in  northern  Indiana,  or 
about  the  time  the  wheat  is  heading.  Their  aim  at  this  time  is  to 
place  the  eggs  singly  in  the  growing  stem  just  above  the  youngest 
and  most  succulent  joints,  which  are  not  so  covered  by  the  enfolding 
leaf  sheaths  as  to  be  inaccessible  to  them  (see  fig.  8).  Thus  it  is  that 
the  stage  of  advancement  in  the  growth  of  the  wheat  stems  at  the 
time  of  oviposition  of  the  summer  generation  of 
females  determines  whether  the  larvae  will  be  well 
upward  in  the  straw,  and  therefore  removed  after 
harvest,  or  lower  down  and  consequently  left  in 
the  field  in  the  stubble. 

The  method  of  oviposition  and  the  point  where 
the  egg  is  usually  inserted  are  shown  in  figure  8. 
The  shape  of  the  egg  after  being  placed  in  the  stem 
is  also  shown  in  the  same  figure.  The  larva  forms 
no  gall,  nor  does  it  harden  the  stem  within  which 
it  develops.  There  is  normally  but  one  larva  in 
each  joint;  but  if  several  eggs  have  been  placed 
between  joints  and  produce  larva'  there  will  be 
one  in  the  center  of  the  stem  just  above  the  joint 
and  others  in  the  walls  just  under  the  internal 
wall  covering  or  inner  epidermis.  These  larva1  in 
the  walls  of  the  straw  do  not,  as  a  rule,  kill  the 
stem,  but  their  effeet  is  to  shrink  kernels,  curtail- 
ing the  yield  by  reducing  the  weight.  The  larvae 
develop  rapidly  and  reach  their  full  growth  before 
the  straw  has  hardened.  By  October,  in  the  Mid- 
dle West,  though  earlier  in  the  South,  they  pass 
into  the  pupal  stage,  in  winch,  as  a  rule,  they  remain  until  early 
spring,  whereupon   they  develop  to  adults  and  gnaw  their  way  out. 


Fir,.  .">, — Larva  of  the 
wheat  striuvworm  (Iso- 
soma  grande):  e.  An- 
tenna; /,  jaw.  Line  at 
right  indicates  natural 
length.     (After  Riley.) 


HISTORY    OF    THE    SPECIES. 


In  June,  1880,  Mr.  J.  K.  P. 
wheat  straws  containing  larvae 
Government  Entomologist,  w 
straw  in  his  field  was  infested 
was  inclined  to  fall  before  the 
these  straws  that  the  types  of 
October,  and  later  on  in  1881, 

[Cir.  106] 


Wallace,  of  Andersonville,  Tenn.,  sent 
of  this  species  to  Dr.  C.  V.  Riley,  then 
ith  the  statement  that  nearly  every 
by  similar  larva1  and  as  a  consequence 
grain  had  fully  ripened.  It  was  from 
Isosoma  tritici  Riley  were  secured.  In 
Mr.  J.  G.  Barlow,  of  Cadet,  Mo.,  sent 


Doctor  Riley  man)  infested  straws,  from  which  adults  of  the  spring 
form  were  afterwards  reared.  At  Carbondale,  III.,  in  June,  1882, 
Prof.  (i.  II.  French  found  the  species  infesting  wheal  fields,  in  Bome 
cases  '.,::  per  cent  of  the  Btraws  being  affected  and  from  one  to  three 
larvffl  being  found  in  «-:i«-l i  Btrav. 

September  L6,  [882,  straws  containing  pups  and  an  occasional 
larva  were  received  bj  Doctor  Rile)  from  Mr.  J  V  Starner,  Dayton, 
Columbia  County,  Wash.  On  October  I,  1883,  the  senior  author 
received  infested  Btraws  from 
Stockton,  Cal.,  through  Professor 
French,  and  from  these  he  was 
alilt<  to  rear  the  spring  form  of 
adults  during  the  following  Febru- 
ary .  On  Ma)  8,  1884,  In-  found 
adults  abroad  in  a  wheal  field  near 
Bloomington,  111.  On  Ma)  9 
oviposition  was  observed,  larva? 
were  found  in  the  wheat  plants 
28,  and  a  single  pupa  was 
found  on  the  following  day,  all  in 
the  same  field  of  growing  wheat. 
Straw  taken  from  thi-~  same  field 
early  in  the  following  July  pro- 
duced the  spring  form  nf  this 
species  the  following  spring 
April,  1885. 

\  Oxford.  Ind..  dune  6,  L884, 
the  senior  author  found  large 
w  inged  adults  of  the summei  form 
ovipositing  in  the  stems  of  grow- 
ing wheat,  and  in  a  small  devel- 
oped -tem  like  those  observed  in 
May,  about  Bloomington,  III.,  a 
living  pupa  Was  found.  Straws 
from  the  Oxford  field  gave  pre- 
cisely the  same  form  the  following  April  as  did  the  -traw  from 
Bloomington.     The  larger  form,  observed   at    Oxford  dune  <>  and 

later.  WAS  also  collected  in  fields  nhout  Bloomington  a  few  days 
earlier.  Larva1  of  this  larger  form  were  found  in  wheat  held-  in 
southern  Illinois  in  May,  and  the  adults  were  observed  in  the  same 
tion  of  country  in  late  May  and  early  June  by  Prof.  11.  Garman, 
at    that    time   assistant    to    Dr.  S.   A.    Forbes.*      Doctor    Riley   de- 


«  here 
big  jwsition  of  egg 


■  Prairie  Farmer,  Jurj  8,  1882 

6  Fourteenth  Rep.  State  Km.  111..  L885 


[Clr.  i<».;| 


scribed  this  larger  form  as  a  new  species,  giving  it  the  name  grande.a 
During  the  years  1884  and  1885,  however,  the  senior  author 
reared  Riley's  Isosoma  tritici  from  straws  in  which  only  his  /. 
grande  had  oviposited  and  his  /.  grande  from  plants  to  which  only 
/.  tritici  had  access.     As  this  last  name  had  been  applied  by  Doctor 

Fitch  many  years  earlier  to  an- 
other insect,  the  jointworni,  the 
name  grande  must  necessarily  be 
given  to  both  forms. 

BUREAU      NOTES     AND     OBSERVA- 
TIONS   BY    ASSISTANT  8. 

Strangely  enough,  one  of  the 
earliest  reports  of  tins  species 
came  from  Mr.  J.  A.  Starner, 
Dayton,  Columbia  County,  Wash. 

Infested  straws  containing  a  few 
larva1,  but  mostly  pupae,  were 
received  September  16,  1882, 
showing  that  even  at  that  early 
date  it  was  sufficiently  abundant 
to  attract  the  attention  of  fa  lin- 
ers. Mr.  D.  W.  Coquillett  found 
it  at  Anaheim  and  Atwater,  Cal., 
in  1SS5,  while  Mr.  Albert  Koe- 
bele  found  it  during  the  same 
year  at  Folsom,  Cal.  It  was 
also  sent  to  the  Department  by 
Mr.  J.  F.  Donkin,  from  Grayson, 
Cal.,  during  the  same  year.  In 
1885  Prof.  F.  H.  Snow  reported 
it  as  doing  serious  injury  in 
McPherson,  Morris,  Osborne. 
Ottawa,  and  Saline  counties, 
Ivans.,  and  mentioned  it  as  a 
new  pest,6  and  in  1S91  it  was 
prevalent  in  central  and  west- 
ern Kansas.''  In  1886  Mr.  Coquillett  found  it  at  Los  Angeles,  Cal. 
During  August,  1890,  infested  straws  from  Washington  State  were 
received  at  the  Department  from  Moses  Bull,  Pullman;  J.  W.  Jessup, 
Rosalia;    G.    W.    Dunn,  Tekoa ;    and   Milton   Evans,  Walla    Walla. 

a  Bul.  Brooklyn  Km.  Soc.,  KM.  pp.  II    L2. 
b  Monthly  Rep.  Kans.  Bd.  Agr.,  June,  L885. 
c  Loc.  (it .,  Feb.,  1892. 
[Clr.   L06] 


Fig.  7. — Isosoma  grande:  Pupa  of  summer  form  in 
young  wheat.    I  <  Original,) 


Although  there  were  no  reports  of  damage  in  the  spring,  injur}  was 

Bufficientrj   marked  to  attract   the  attention  of  farmers,  and  f i 

these  one  frequently  hears,  even  up  to  the  present  time,  descriptions 
and  reminiscences  of  what  Beema  to  have  1  mm-h  the  first  serious  out- 
break of  the  pesl  in  the  United  States. 

The  senior  author  found  the  species  abundant  but  not  destructive 
ni  Princeton,  Ind.,  in  1887  and  again  in  1902.  June  27,  1893,  int 
straws  were  sent  to  the  Department  of  Agriculture  bj  Mr.  E.  J. 
Woodville,  [ndiantown,  Va.  In  1905  the  junior  author  found  the 
species  infesting  growing  wheat  about  Conway,  Ark.,  April  7.  the 
larvae  being  apparentlj  full  grown;  and  at  Petty,  Tex.,  April  I  l.  he 
found  larvae  and  pupaa  present  in  great  numbers,  in  some  cases  ">o 
per  cciii  of  tli<'  young  wheal  plants  being  infested      He  also  found 

half  -  grown     larvae     ;it     Concordia,        

-.,  April  21 .  During  the  same 
year  the  senior  aul  bor  found  Ian  as 
in  young  wheat  at  Lexington,  N. C, 
April  12;  and  at  Statesville,  V  C  . 
on  t  hr  follow  ing  day,  he  found  not 
only  larvae  in  the  young  plants,  l>ut 

femalesof  the  spring  form  in  tli t 

of  ovipositing.  At  Charlotte,  N.  C, 
April  l  I,  larvae  and  pupae  were  found 
abundantly  in  the  yoimg  w  heat .  ( )n 
the  imIi  of  the  same  month  young 
larvae  were  found  in  young  wheat 
plants  at  Dublin,  Va.  Mr.  W.  J. 
Phillips  found  the  species  quite  nu- 
merous in  young  wheat  plants  at 
Nashville,  Tenn.,  April  20,  and 
studied  the oviposition of  bothforms 
at  Richmond,  Ind.,  from  May  L'ti  to 
June  _'7.  June  1 6  t  he  Bureau  received  infested  31  raws  from  Mr.  1 1 .  W. 
Joy,  Hays,  Cans.  On  May  2  I  of  the  following  year.  Mr.  Phillips  found 
the  species  abundanl  at  Geneva,  Ind..  where  the  females  of  the  summer 
form  were  jusl  beginning  to  emerge  from  the  young  wheat;  and  the 
senior  author  observed  it  again  at  Charlotte,  V  ( '..  May  9,  with  indica- 
tions that  it  had  done  considerable  injury  to  young  wheat.  Mr. 
Phillips  observed  it  during  1907  at  Kingfisher,  Okla.,  and  at  Beloit, 
Elans.,  on  April  15;  at  Lenora,  Kans.,May20;  and  at  Kearney,  Nebr., 
Ma\  24.  In  all  cases  it  was  more  or  less  abundant,  dune  1  of  the 
same  year  Mr.  ('.  \.  Ainslie  found  adult-  of  the  spring  form  at 
Wellington,  Kan-. ;  from  April  :>0  to  May  7  he  found  the  adult  females 
of  this  form  abroad  in  the  fields  at  Sterling,  Kan-. ;  and  on  May  '_'•'•  he 
found, them  ovipositing  in  wheat  ai  Hay-.  Kan-.,  and  other  points 

[fir.  106] 


" 
of   summer    form  'It:    a. 

Won  In 

I,  AftiT 


8 

farther  west.  He  found  them  ovipositing  in  durum  wheat  at  Oakley, 
Kans.,  June  3,  and  on  the  following  day  also  in  durum  wheat  at 
Colby,  Kans.  Again,  June  8,  he  found  them  abroad  in  wheat  fields 
at  Manhattan,  Kans.  Females  of  the  summer  form  were  observed  by 
Mr.  E.  O.  G.  Kelly  ovipositing  at  Caldwell,  Kans.,  May  14,  1908. 

May  26  to  28,  1908,  Mr.  Ainslie  found  both  sexes,  the  females  ovi- 
positing, near  Chambersburg,  Pa.  This  is  the  farthest  east  that  we 
have  any  record  of  the  existence  of  the  species,  the  most  eastern 
point  of  occurrence  previously  known  being  at  Andover,  in  extreme 
northeastern  Ohio,  where  the  senior  author  found  the  larvae  hi  wheat 
straw,  August,  1904. 

As  will  be  observed  from  the  foregoing,  the  adult  female  emerges 
early  in  spring  and  deposits  her  eggs  in  the  young  wheat  plants  before 
the  latter  have  made  much  growth.  By  the  time  wheat  is  beginning 
to  show  the  heads  within  the  enfolding  sheath  leaves,  adults  of  the 


Fig.  9.—Mu,pelmus  aUynii:  Male,  much  en- 
larged.   (From  Riley.) 


Fig.  10. — Eapelmus  aUynii:  Female,  much  enlarged. 
I  From  Riley 


second  generation  are  abroad  and  depositing  in  the  upper  steins;  the 
larva3  from  these  pass  the  winter  as  pupae  and  emerge  as  adults  early 
the  following  spring,  exact  dates  of  course  varying  with  the  latitude 
and  other  influences. 


NATURAL    ENEMIES. 


Probably  the  most  efficient  enemy  of  this  species  is  a  small,  slender, 
four-winged  fly  (figs.  9,  10),  with  a  somewhat  brilliant  metallic  body 
and  yellow  legs.  It  has  a  very  slight  resemblance  to  an  Isosoma,  and, 
indeed,  was  described  by  French  as  Isosoma  aUynii,  but  it  is  now 
known  as  Ewpelmus  aUynii.  A  somewhat  similar  insect  with  metallic 
body  and  yellow  abdomen,  SemioteUus  isosomatis  Riley  (fig.  11),  is 
very  efficient  in  destroying  the  larva3  in  the  straw.  Ilomoporus 
(SemioteUus)  chalddiphagus  Walsh  (fig.  12),  and  beyond  a  doubt 
other  chalcidids,  are  also  instrumental  in  holding  it  in  check.     These 

[Clr.  L06] 


parasites   are   all   the   more  efficient    as   the}    are   double-brooded, 
developing  in  late  summer  and  at  once  ovipositing  in  other  larvae. 

In  1908,  al  Wellington,  Bums.,  Mr.  K.  O.  G.  Kelly,  beginning  April 
10  with  the  larvae  in  the  st  ram  .  followed  the  insect  up  to  June  I,  wit- 
nessing oviposition  in  both  the  spring  and  summer  forms.  Through 
the  remainder  of  June  and  most  of  Jurj  he  was  absent,  and  on  his 
ret  urn  he  was  unable  to  find  the  larvae  in  the  straw,  although  stubble 
nil 1 1  si  raws  perforated  \\  ii  li  holes  made  bj  the  escaping  parasites  were 
found  everywhere  in  the  fields.  On  account  of  the  abundance  of  such 
evidence  in  the  fields,  Mr.  Kelly  was  convinced  that  the  pest  had  been 
largely  overcome  in  that  locality  by  E/updmus  allynii.  At  Pratt, 
Kan-..  November  17,  a  few  larvae  were  found  in  wheal  straws,  with 
indications  that  parasites  had  been  present  to  a  limited  extent,  and 
practically  the  same*situation  was  found  to  occur  at  Sedgwick,  Kans., 
the  following  day.  Quite  significant,  however,  was  the  fact  that  in 
the  vicinit}  of  Prat  t,  on  Sept  em  Iter  16,  Mr.  T.  1).  Urbahns  found  in  an 


Pie.  i 


I 


old  wheal  field  not  a  single  living  fsosoma  larva,  while  85  per  cent  of 
the  straws  gave  unmistakable  evidence  by  the  holes  in  them  that 
these  larva'  had  been  present,  hut  were  destroyed  by  parasites. 

Quite  in  accord  with  this  it  will  be  remarked  that  the  junior  author, 
in  bis  discussion  of  the  outbreak  of  the  pesl  in  Washington  Mate,  in 
1908,  does  not  mention  parasites  at  all  not  because  he  overlooked 
the  matter,  hut  because  he  reared  only  a  single  individual  parasite, 
probably  Semiotettus  isosomatis  Riley,  ami  there  was  no  evidence  "I 
the  presence  of  parasites  in  any  numbers  in  the  section  of  country 
visited  by  him.  This  will  perhaps  throw  some  tighl  upon  the  prime 
causes  of  this  outbreak.  Besides,  it  opened  the  w,i\  for  an.inti 
duction  into  Washington  State  of  the  parasites  affecting  the  pest  in 
the  Bast.  It  i-  as  yet  to.,  soon,  however,  to  expect  definite  results 
from  these  introductions. 

In   Indiana  and  the  Middle  West   when  the  wheat    i-  harvested  the 

straw  is  frequently,  and.  in  fact,  almost  invariably,  cm  <,ir  between 

Kir 


10 


joints,  thus  leaving  the  larvae,  if  there  are  such  in  the  straws  at  that 
point,  exposed  to  attack  from  predaceous  insects.  The  larvae  of  a 
small,  slender,  black  and  yellow  carabid  beetle  (Leptotrachelus  dor- 
salis  Fab.)  crawls  up  the  stalks,  descends  into  the  stubble,  and  devours 
t  he  Isosoma  larva?,  but  unfortunately  its  taste  is  such  that  it  docs  not 
confine  itself  strictly  to  Isosoma  but  devours  parasite  as  well  as  host. 
The  mite  Pediculoides  (Hetero'pus)  ventricosus  (fig.  13)  is  also  an 
enemy,  gaining  access  to  the  larvae  precisely  as  do  the  beetle  larvae 
previously  mentioned. 

PREVENTIVE    MEASURES. 

There  are  no  remedial  measures,  since  it  is  impossible  to  alleviate 
or  influence  in  any  way  the  effect  of  larvae  in  the  infested  wheat  plants. 
As  it  is  the  larger  and  more  vigorous  plants  that  the  females  select  for 
egg-laying  purposes,  no  amount  of  enriching  the 
soil  will  affect  the  numbers  of  the  pest  or  lessen  their 
influence.  Preventive  measures  are,  however,  sim- 
ple, practicable,  and  effective.  Indeed,  the  wheat- 
straw  worm  is  one  of  the  very  easiest  of  all  insects 
to  control.  The  diminutive  size  of  the  female  that 
emerges  in  early  spring  renders  extended  migrations 
fatal,  since  during  that  period,  when  the  weather  is, 
of  all  seasons,  the  most  erratic  and  liable  to  sudden 
and  extreme  changes,  it  tries  to  avoid  the  frequent 
storms  by  seeking  protection  on  the  underside  of  the 
leaves  of  the  growing  plants.  But,  most  fortunate 
of  all  for  the  farmer,  large  numbers,  if  not  indeed 
much  the  greater  proportion,  of  these  females  have 
no  wings  and  can  only  make  their  way  from  place 
to  place  by  crawling. 
The  advantage  of  all  of  this  to  the  farmer  is  shown  by  the  fact  that, 
where  the  preceding  crop  has  been  other  than  wheat,  the  injury  to  the 
young  wheat  plants  in  spring  is  confined  to  the  margins,  and  especially 
where  such  margins  border  on  other  fields  that  have  been  devoted  to 
wheat  for  a  number  of  consecutive  years.  Mr.  Reeves,  as  will  be  ob- 
served further  on  in  this  publication,  has  been  able  to  measure  the 
relative  diminution  of  attack  from  the  borders  of  the  field  inward 
toward  the  center.  Similar  observations  had  previously  been  made 
in  other  sections  of  the  country.  One  of  the  earliest  notes  made  on 
this  species  by  the  senior  author  was  in  June,  1884,  in  a  field  of  wheat 
sown  the  previous  autumn,  among  corn,  the  land  bordering  to  the 
south  on  a  stubble  field.  The  following  spring  there  were  many 
wheal  plants  injured  along  the  south  margin  of  the  field,  but  none 
could  be  found  along  the  northern  border.     With  the  appearance  of 

[Cir.  106] 


Fig.  Vl.—Pcdiculoidfs 
venlricosus  Newp.,  a 
mite  which  destroys 
the  larva.  Much 
enlarged.    (After 

Marl 


II 

the  seconil  generation,  composed  of  large,  vigorous  females  with  well 
developed  wings,  the  diffusion  would  become  comparatively  general, 
and,  therefore,  the  second  year,  If  the  held  were  to  be  continued  in 
wheat,  would  find  the  infestation  more  nearly  uniform  throughout. 

li  is  significant,  t"",  thai  the  pesl  becomes  much  more  destructive 

the  longer  w  heal  is  groM  n  cont  inuousl)  on  the  sa ground.     As  early 

as  1882  Prof.  <i.  II.  French  observed  thai  while  93  per  cenl  of  the 
straws  were  infested  in  a  field  where  the  same  grain  had  been  grown 
the  year  before,  in  a  field  where  clover  had  been  grown  the  previous 
year  only  aboul  5  per  cenl  of  the  straws  were  infested.  At  Petty, 
Tex.,  April  I  I.  1905,  the  junior  author  found  a  field  of  wheal  thai  had 
been  in  this  grain  for  two  consecutive  years  very  seriously  injured. 
There  were  spot-  several  yards  square  where  the  ground  was  bare, 
and  surrounding  these  were  an 'a-  where  half  of  the  living  plants  were 
infested.     At  Sawyer,  Kan-.,  in  an  old  field,  Mr.  Urban ns  found  that 

per  cenl  of  the  straws  gave  unmistakable  evidence  of  having  been 
in IV- ted.  All  of  this  points  conclusively  to  the  efficiency  of  a  rot  at  ion 
of  crops  that  will  eliminate  the  grow  ing  of  v.  heat  two  years  in  succes* 
si.m  on  the  same  land.  When-  for  any  reason  this  can  nol  be  dour, 
tlit*  measure  nexl  in  efficiency  will  be  the  burning  of  the  stubble  and 
outstanding  straw  during  the  fall  or  winter. 

RELATIONS    OF    THE   WHEAT   STRAWWORM   TO   WHEAT    PRODUCTION    IN 

THE  NORTHWEST 

By  Geo    I    R 

The  wheat  -t  raw  worm,  by  reason  of  h-  relation  to  both  the  climate 
and  the  farm  method-  of  the  mosl  productive  portion  of  the  Pacific 
Northwest,  is  capable  of  great  injury  to  wheat  growers.  Though 
it-  greatest  inroad-  are  made  upon  spring  wheat,  it  is  winter  and 
volunteer  wheal  which  render  those  inroad-  possible.  It-  work  is 
of  such  nature  that  it  passes  unobserved,  mile—  an  attack  of  unusual 
erity  causes  the  farmer  to  scrutinize  his  fields  more  closely  than 
he  is  accustomed  to  do.  The  constant  small  losses  occasioned  by 
this  insect  and  the  possible  destruction  of  the  crop  over  considerable 
area-  may   be  prevented   by  such   measures  as  every  good  farmer 

employs  to  maintain  the  fertility  of  his  -oil.  9ecure  a  good  - I  bed, 

retain  moisture,  and  eradicate  weeds. 

The  strawworm  may  be  found  wherever  wheat  is  grown  in  the 
Columbia  basin,  thai  great  and  fertile  plain  of  varying  topography 
and  climate  which  i-  inclosed  by  the  Okanagan  Highlands  on  the 
north,  the  Rocky  Mountain-  on  the  east,  tin1  Blue  Mountain-  on  the 
south,  and  the  Cascade  Mountains  on  the  west;  bul  it  is  not  equally 
destructive  over  all  thai  territory.  The  central  and  western  parts 
of  this  basin,  except  iii  the  (ioldcndale  and  Horse  Heaven  districts, 

[fir    106] 


12 

are  arid,  producing  little  or  no  wheat,  and  a  strip  cast  of  this,  known 
as  the  dry-farming  region,  where  the  scanty  rainfall  makes  it  nec- 
essary to  summer-fallow  the  land  every  second  year,  is  by  that 
means  protected  from  serious  injury;  but  the  eastern  zone,  a  strip 
200  miles  long,  included  between  the  dry -farming  area  and  the  upper 
limit  of  wheat  culture  at  the  elevation  of  3,000  feet,  affords  ideal 
conditions  for  the  production  of  small  grains  and  also  for  the  prop- 
agation of  this  insect.  The  climate  is  characterized  by  mild  winters 
suitable  for  the  growing  of  winter  wheat — cold,  wet  spring  weather, 
such  as  is  desirable  for  all  small  grains,  and  hot,  dry  summers  per- 
fectly adapted  to  the  maturing  and  harvesting  of  the  crop.  The 
soil  is  such  that  after  more  than  thirty  years  of  almost  continuous 
production  it  still  yields  30  to  40  bushels  of  wheat  per  acre.  These 
conditions  encourage  the  seeding  of  wheat  year  after  year  upon  the 
same  ground,  and  such  a  practice,  together  with  the  presence  of 
winter  or  late  volunteer  wheat,  supplies  the  requirements  for  the 
unhindered  multiplication  of  the  strawworm.  Owing  to  the  drought 
of  summer  the  volunteer  wheat  usually  does  not  germinate  until 
about  the  same  time  as  the  fall-sown  wheat,  and  the  significance  of 
this  fact  appears  when  we  come  to  study  the  insect's  life  history. 

This  Isosoma  passes  the  winter  in  the  larval  (fig.  1)  or  pupal 
(fig.  7)  stage  and  reaches  maturity  early  in  April.  It  gnaws  its 
way  out  of  the  stubble  in  which  it  has  hibernated  and  deposits  its 
eggs  in  the  heart  of  the  young  plants  of  winter  or  volunteer  wheat 
(fig.  6),  which  must  be  near  the  place  of  emergence,  since  this  early 
generation  is  largely  wingless  (fig.  3),  and  its  effective  range  as 
measured  by  the  spread  into  an  adjoining  field  is  not  much  more 
than  12  feet.  The  food  plants  of  the  early  generation  may  be  sup- 
plied by  winter  wheat  or  by  volunteer  wheat  not  too  far  advanced 
to  attract  the  insect.  The  oviposition  is  finished  before  any  spring 
wheat  is  up,  so  that  that  crop  suffers  no  attack  from  the  first  genera- 
tion. The  winter  and  volunteer  wheats  lend  an  essential  aid  to  the 
perpetuation  of  the  species. 

The  larva  (fig.  5),  when  it  hatches  from  the  e^,  is  surrounded  by 
the  most  delicate  tissues  of  the  plant  and  feeds  upon  them,  destroy- 
ing the  young  head  and  thus  excavating  a  residence  to  accommodate 
its  rapidly  increasing  size.  The  outer,  tougher  layers,  which  it 
does  not  attack,  gradually  become  fleshy,  stiff,  and  brittle,  forming 
a  hollow,  bulblike  swelling  at  the  base  of  the  plant.  This  enlarge- 
ment is  discernible  from  the  outside  of  the  infested  plant,  and  its 
presence  may  be  verified  by  squeezing  the  crown  of  the  plant  be- 
tween the  lingers,  whereupon  the  bull)  at  first  firmly  resists  pressure 
and  then  breaks  open,  disclosing  the  larva.  The  infested  plant 
suffers  a  change  in  external  form,  which  causes  it  to  resemble  wheat 

[Cir.  10C] 


13 

injured  l>\  the  Hessian  fly  and  by  Meromyza,  but  it  may  usually 
be  distinguished  from  the  former  by  the  curly  and  slightU  flesh} 
form  of  the  blades;  from  the  latter  l>\  the  absence  of  the  brown, 
threadlike  central  Bhoot,  as  well  as  the  form  "f  the  blades;  and  from 
l>"th  1>\  the  presence  of  the  crisp  l>ull>  at  the  crown.  The  lower 
blades  are  broad,  dark  bluish  green,  curl}  .  and  slightly  fleshy,  because 
they  arise  between  the  rool  and  the  point  of  injury  and  thus  receive 
the  Bap  that  is  being  diverted  l>\  the  larva  from  the  main  growing 
point  of  tin'  plant.  The  plant,  when  attacked  thus  earl)  at  the 
heart,  remains  low,  does  not  produce  any  head,  and  perishes  when 
the  lower  blades  of  the  plants  naturally  die,  about  the  middle  of 
June.  At  about  the  Bame  time  the  adults  of  the  BecoriH  genera- 
tion (fig.  H  begin  to  emerge  from  these  plants  and  deposit  eggs 
upon  the  wheat.  These  insects,  as  were  those  of  the  first  generation, 
are  nearly  all  females,  and  the}  also  reproduce  withoul  the  assist- 
ance of  males,  Imi  they  differ  from  the  earlier  form  in  being  lai 
and  in  possessing  wings,  which  enable  them  to  attack  fields  at  a 
distance  from  their  origin.  Thej  prefer  the  younger  plants,  and  as 
the  Bpring  wheat  is  small  at  this  time,  while  the  winter  wheat  is  old 
and  tough,  the  Bummer  attack  is  concentrated  upon  Bpring  wheat. 
if  there  be  any  in  the  vicinity.  Thej  attack  it  in  much  the  Bame 
way  that  the  spring  generation  employs  upon  the  young  winter 
wheat,  and  with  the  Bame  result;  the  smaller  plants  simulate  a 
luxuriant  growth,  but  remain  short  and  soon  die:  the  larger  ones 
may  produce  heads,  a  few  inches  above  ground,  but  such  heads 
contain  no  kernels.  Thirty  acre-  of  spring  wheat  near  ( lolfax,  Wash., 
were  completely  ruined  in  this  manner  in  the  summer  of  1 

Since  spring  wheat  is  not  so  extensively  grown  as  formerly  in  this 
region,  the  summer  adults  are  often  compelled  to  oviposit  in  winter 
wheat,  and  in  those  cases  the  plant  i-  le—  severely  injured,  because 
it  is  old  and  tough;  hut  the  larva  of  the  insect  and  the  resulting 
adult  take  the  same  form  and  size  w  hether  they  develop  in  the  tender 
Bpring  wheat  nr  in  the  more  mature  winter  wheat.  The  succulence 
of  the  former  does  not  cause  an  increase  in  the  size  of  the  larva'  which 
feed  upon  it.  nor  does  it  produce  a  larger  proportion  of  winLr<'d  indi- 
viduals among  the  adults.  On  the  contrary,  it  seems  that  austere 
and  unfavorable  circumstances  tend  to  increase  the  numher  of  winged 
adults,  as  a  large  percentage  of  those  reared  from  stubble  collected 
at  Goldendale,  St.  Germain,  and  Waterville,  Wash.,  in  the  arid  dis- 
trict, have  wings.  The  egg  is  placed  in  a  tender  joint  near  the  In 
(fig.  8)  and  the  larva  hatches  and  gnaws  the  inner  surface  of  the 
stalk,  too  late  to  prevent  the  formation  of  the  head,  hut  early  enough 
to  shorten  it  and  hinder  the  filling  of  the  kernels.  To  ascertain  the 
amount  of  this  injury  1,452  heads  taken  from  an  apparently  unin- 

[Cir.  106] 


14 

jured  field  of  winter  wheat  near  Pullman,  Wash.,  were  dried  and 
weighed.  Thirty-nine  per  cent  of  these  were  afterwards  found  to 
have  grown  upon  stems  damaged  by  the  strawworm,  and  the  average 
weight  of  the  J  leads  from  these  damaged  stems  was  22  per  cent  below 
the  average  of  healthy  heads,  although  no  difference  in  size  was 
noticeable  until  the  heads  were  placed  side  by  side,  when  it  was  found 
that  most  of  the  larger  heads  were  those  from  uninjured  plants, 
while  most  of  the  shorter  ones  were  from  infested  plants.  Between 
heads  of  the  same  size  there  was  a  difference  of  7  per  cent,  which 
could  not  be  detected  except  by  weighing. 

Both  the  work  of  the  spring  brood  in  fall  wheat  and  that  of  the 
summer  brood  in  spring  and  winter  wheat  are  so  carried  on  that  the 
owner  does  not  knowrof  Ins  loss  unless  it  amounts  to  a  very  large  per- 
centage of  the  crop,  and  even  then  he  may  attribute  it  to  some  other 
cause.  The  presence  of  a  large  number  of  unusually  rank  and  appar- 
ently sturdy  plants  among  the  winter  wheat  does  not  seem  alarming, 
but  the  reverse ;  and  there  is  no  other  indication  of  work  of  the  spring 
brood.  Later,  when  these  infested  plants  begin  to  die  they  are  con- 
cealed by  the  yellowing  blades  of  the  healthy  plants,  so  that  even  the 
trained  entomologist  can  hardly  find  them.  The  same  is  true  of  the 
spring  wheat  which  is  attacked  by  the  summer  brood,  while  winter 
wheat  plants  attacked  by  the  summer  brood  are,  as  we  have  just 
shown,  so  little  altered  in  appearance  that  the  presence  of  the  insect 
can  be  detected  only  by  weighing  the  heads  or  splitting  the  stems  to 
look  within  for  the  larvae. 

Many  of  the  larvae  remain  in  the  stubble  if  the  grain  is  cut  with  a 
harvester,  and  most  of  them  if  it  is  cut  with  a  header.  They  begin 
to  pupate  during  the  latter  half  of  July,  but  many  remain  in  the  larval 
condition  until  May  of  the  following  year. 

The  life  history  just  related  shows  that  strawworm  injury  is  in- 
duced by  growing  crops  of  winter  wheat  repeatedly  upon  the  same 
"round;  by  leaving  volunteer  plants  among  the  spring  wheat;  by 
allowing  summer  fallow  to  grow  foul  with  volunteer  wheat,  even  at 
a  distance  from  wheat  fields ;  and  by  growing  spring  wheat  near  winter 
wheat.  All  of  these  cases  occur  at  times  in  the  wheat  country  wliich 
we  are  considering.  The  essential  conditions  in  each  of  these  cases 
are,  first,  the  presence  of  infested  stubble,  plowed  or  unplowed,  to 
furnish  the  early  adults;  second,  a  growth  of  winter  or  volunteer 
wheat  early  in  the  spring  to  receive  the  eggs  of  the  first  generation; 
third,  a  crop  of  winter  or,  preferably,  spring  wheat  to  receive  the  eggs 
of  the  second  brood.  The  remedy  lies  in  avoiding  these  conditions 
by  rotation  of  crops,  clean  early  summer  fallowing,  and  the  aban- 
donment of  spring-wheat  culture.  Fall  plowing  ol  the  stubble  does 
not  prevent  the  insects  from  coming  out  and  attacking  wheat  in  the 
[Cir.  106] 


1. 


spring.     Burning  the  stubble  i-   impracticable,  because   the   joi 
uliicji  contain  the  larvte  remain  sappj  during  the  fall  and   will  not 
burn.     Rotation,  to  be  effective,  must  b  •  pi  inned  with  r<  ference  to 
the  work  of  the  insect,  since  two  adjoining  fields  which  are  in  wheal 
in  alternate  years  have  between  them  a    trip  2  rods  wide  which  alwi 
furnishes  ideal  conditions  for  its    pread.     The  stubble  in  one  field 
always  furnishes  insects  to  attack  the  growing  wheat  in  the  otl 
and  while  the  damage  done  !»\  the  wingless  first  generation  U  limited 
to   the    l-rod  strip   next    the   fence,   the   winged   Becond   generation 
spread-  at  \\  ill  i  hrough  the  field. 

There  is  uo  reason  whj  an  insect,  so  readily  held  in  check  merer) 
h\  careful  farm  methods,  should  cause  constant  loss  and  an  occasional 
menace  to  the  wheat  country  of  the  Northwest,  bul  there  is  no  doubt 
thai  it  does  so. 

Approved : 

.1  kMES    Wn  BON, 

Secretary  of  Agricutiun . 

\Y  1SH1NGTON,    1).  (  ..  April  JO,  19 

100J 

o 


UNIVERSITY  OF  FLORIDA 


3  1262  09216  5140 


